This invention relates to improvements in electric motors in power assisted steering systems for vehicles in which the motor is adapted to apply an assistance torque to a portion of steering shaft in response to a motor drive signal. The invention also relates to electric power assisted steering systems of the kind set forth.
In one known power assisted steering system, a first portion of the shaft is operatively connected to a steering wheel, a second portion of the shaft is operatively connected to at least one steerable wheel, a torque sensor is adapted to produce an output signal indicative of torque applied to the first portion of the shaft by a driver of the vehicle, and control means are adapted to process the output signal in order to produce the motor drive signal.
The first and second portions of shaft may comprise respective portions of a single shaft, or may each comprise a separate shaft.
The purpose of an electric power assisted steering system of the kind set forth is for the motor to apply an assistance torque to the second portion of shaft which makes it easier for the driver to turn the steering wheel. Typically, the motor is controlled using a pulse width modulation strategy with the motor drive signal being proportional to the torque required from the motor.
In the prior art, such as our earlier application GB 9619175 the motor comprises a brushless motor driven by an electrical circuit in which the motor phase windings are connected in a bridge. Each phase is connected into a separate arm of the bridge between a positive and negative (or ground) supply lines. A respective top switching device is connected in series in each arm between a phase winding and the positive supply, and a respective bottom switching device is connected in each arm between a phase winding and the negative (or ground) supply line. Thus, for a 3-phase motor the bridge has 3 arms, each comprising a top and a bottom switching device. The motor can be controlled by pulse width modulating the top and bottom switching devices in the arms in any known manner to cause the motor to rotate.
A problem with brushless permanent magnet motors is that a failure mode exists in which the motor phase windings or parts thereof may be shorted together to form a loop through which an induced current can flow. This typically occurs if two top and/or two bottom devices stick or otherwise fail in a closed state to form a closed path around the electrical circuit. In this failure mode a high braking torque may be generated which may make the steering wheel difficult to turn.